Thermally conductive channel between a semiconductor chip and an external thermal interface

ABSTRACT

An apparatus is described comprising a chamber containing liquid. A side of the chamber is thermally coupled to a semiconductor chip. The side of the chamber has thermally conductive carbon nanotubes oriented perpendicular to the side&#39;s surface. The carbon nanotubes transfer heat drawn from the semiconductor chip into the liquid, causing it to boil and spread heat laterally across the top face of the chamber. The top face of the chamber may be thermally connected to an external heat sink if necessary. This device allows for a greatly improved ability to transfer heat from the hot spots of a semiconductor device to the ambient medium.

FIELD OF THE INVENTION

The field of invention relates generally to heat removal; and, more specifically, to an improved thermally conductive channel between a semiconductor chip and an external thermal interface

BACKGROUND

The power consumption of electrical circuitry has emerged as, perhaps, the single largest threat to the continued advancement of semiconductor technology and its ability to craft new markets through the shrinking of transistor device size. Simply put, the smaller a transistor can be made, the more power will be consumed per transistor (owing to the transistor's faster speed and substrate leakage) and the more transistors can be fit onto a single chip of silicon. The combination of more transistors per chip and greater power consumption per transistor has resulted in some of the more advanced semiconductor chips under development exhibiting excessive heat dissipation.

Therefore, semiconductor chip developers are devoting significant resources to the study and development of higher performance yet cost effective chip cooling technologies. Traditionally, cost effective chip cooling has meant “air-cooled” heat sinks.

As a general perspective, chip cooling technologies are actually more accurately viewed as a heat removal systems. Here, heat generated by a semiconductor chip is transferred to an “external thermal interface”; and, then, the external thermal interface “externally” convects, conducts or radiates the heat to some medium (typically air) that is not deemed part of the semiconductor chip and its associated packaging. Here, the ability to transfer heat “externally” from the semiconductor chip and its packaging corresponds, in turn, to its cooling.

According to air-cooled heat sink approaches, the external thermal interface is a heat sink made of thermally conductive fins that rise above the surface of the semiconductor chip's package. The heat dissipated by a semiconductor chip is channeled to the heat sink's fins. As a general rule, cooling efficiency improves as the surface area of a heat transferring material increases. With respect to heat sinks, the fins of the heat sink effectively create an expanded external thermal interface surface area over which the semiconductor chip's heat is externally convected and/or radiated.

When air is blown through the heat sink's fins, heat is transferred from the fins to the air so as to effectively remove heat from the semiconductor chip and its associated packaging. Unfortunately, the traditional air-cooled mechanism described above—although cost effective—may not exhibit sufficient performance for future high performance and/or high density semiconductor chips.

FIGURES

The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:

FIGS. 1 a and 1 b show semiconductor chips and their respective hot spots;

FIG. 2 shows an improved thermally conducting channel for use between a semiconductor chip and an external thermal interface;

FIG. 3 shows an alternate embodiment to that of FIG. 2 in which a liquid flow flows through the chamber;

FIG. 4 shows a cross section of a packaged die.

DETAILED DESCRIPTION

FIGS. 1 a and 1 b attempt to graphically depict a particular challenge that, if overcome, could possibly lead to higher performance yet reasonably affordably chip cooling technologies. The particular challenge is uniformly spreading out heat generated from specific regions of the chip that generate excessive heat (commonly referred to as “hot spots”) as the heat is carried to the external thermal interface.

FIG. 1 a shows a temperature profile of a first semiconductor chip and FIG. 1 b shows a temperature profile of a second semiconductor chip that is differently designed than the first (e.g., the first semiconductor chip may be a microprocessor and the second semiconductor chip may be a memory controller). Concentric rings are observed on both chips. The smaller the region bounded by a ring, the more heat generated within the ring as compared to larger surrounding rings.

Hot spots are generally created by regions of circuitry operating at high speed. Typically, a digital circuit region will tend to generate more heat as its constituent transistors: 1) are packed more densely; 2) operate faster, and, 3) push/pull more current. The smallest rings observed in FIGS. 1 a and 1 b therefore correspond to regions of the respective circuit designs that unfortunately excel in all three factors listed above as compared to the circuit designs' other regions. Comparing FIGS. 1 a and 1 b, concentric rings reside at different locations owing to the different transistor designs designed into the two chips.

Owing to inefficiencies in the thermal channel that exists between a semiconductor chip and its heat sink, present day generically designed heat sink approaches do not respond well to semiconductor chip “hot spots”. More precisely stated, the thermal conductive channel that exists between the external thermal interface (i.e., the heat sink) and the semiconductor chip conducts heat from different regions of the chip differently. As such, certain regions of the semiconductor chip will enjoy lower thermal resistance between themselves and the external thermal interface than other less fortunate regions.

If a “hot spot” happens to reside in a region that does not enjoy lower thermal resistance between itself and the external thermal interface, the effected heat transfer from the hot spot may be insufficient to keep the semiconductor chip within acceptable thermal operating limits. The problem can be lessened at least somewhat by custom designing the thermal conductive channel between the semiconductor chip and the external thermal interface on a chip-design by chip-design basis.

Thus, for example, a first conductive channel could be designed for the semiconductor chip of FIG. 1 a that provides for the lowest thermal resistance at the hot spot regions observed in FIG. 1 a, and, a second conductive channel could be designed for the semiconductor chip of FIG. 1 b that provides for the lowest thermal resistance at the hot spot regions observed in FIG. 1 b. A problem, however, is that the above solution is essentially a “custom” design for each chip, and, custom implementations tend to be more expensive than generic implementations that are theoretically suitable for any chip.

FIG. 2 provides a depiction of an improved conductive channel that resides between a semiconductor chip 201 and its external thermal interface (e.g., a heat sink that is thermally coupled to layer 214). The improved conductive channel effectively operates by what can be referred to as “uniform condensation”. Here, vapor molecules from a pool of liquid 208 that is heated by any semiconductor chip region condense on the underside 211 of the lid of a chamber that contains the liquid and its vapor. Both the boiling of the liquid and the condensation of its vapor corresponds to a heat transfer process that together remove heat from the semiconductor chip 201.

Importantly, the vapor molecules scatter randomly within the chamber 207. As such, the location where a vapor molecule condenses on the lid underside 211 should be effectively random relative to the semiconductor chip region whose heat nucleated the vapor molecule.

Thus, through this process, heat generated from a particular semiconductor chip region should be uniformly distributed across the lid underside 211. All regions of the semiconductor chip should therefore enjoy approximately the same thermal resistance between themselves and the external thermal interface; and, as a result, custom thermal packaging solutions can be avoided. Immediately following is a more thorough discussion of the principles of operation of the technique depicted in FIG. 2.

According to the depiction of FIG. 2, a semiconductor chip/die 201 with “flip-chip” technology in the form of a ball grid array (which is partially comprised of ball contacts 202) is shown. Thus, the metallurgy and I/O contacts for the device are located on the underside of die 201. Atop the side of the die opposite that of its interconnect metallurgy sits the bottom floor 203 of a chamber that contains a pool of liquid 208. The floor layer 203 includes, akin to a heat-sink structure, vertical studs 213 of thermally conductive material.

Any thermally conductive stud will radiate heat generated by the semiconductor chip in a region of the semiconductor chip 201 that is, approximately, directly beneath the stud. The thermal transfer properties of the thermally conductive studs are similar to those described in the background with respect to heat sink implementations. That is, the efficiency of heat transfer from the semiconductor chip 201 into the liquid 208 is improved because the thermally conductive studs effectively correspond to a greater surface area of the chamber's floor layer 203.

FIG. 2 corresponds to a cross section depiction, and, as an example, cross sections of two regions of circuitry 209 ₁, 209 ₂ that generate hot spots are encircled. Assuming at least some correlation between the heat generated at a particular region of circuitry and the particular layer of the chamber's lower layer 203 where the resulting, transferred heat is observed, it is expected that the studs rising above the chamber's lower layer 203 in chamber regions 210 ₁, 210 ₂ will correspondingly increase the temperature of the liquid 208 to a value that is higher than other chamber regions that do not reside above a hot spot.

For simplicity, these regions 210 ₁, 210 ₂ have been drawn to include nucleated “bubbles” that result from heat being transferred from the studs within regions 210 ₁, 210 ₂ residing above hot spots 209 ₁, 209 ₂ into liquid 208. That is, the liquid inside regions 210 ₁, 210 ₂ have been depicted as boiling above hot spots 209 ₁, 209 ₂. It should be understood that in many practical implementations the design point of operation within the chamber is expected to be simply that the liquid above a hot spot will nucleate bubbles 208 more rapidly than other “non hot spot” regions.

As discussed above in some detail, the vaporization of the liquid 208 above the hot spot regions 209 ₁, 209 ₂ will result in the generation of vapor molecules above the liquid 208 within region 207 of the chamber. Owing to their high kinetic energy, the vapor molecules will effectively travel randomly within chamber region 207, resulting in the condensation of at least some vapor molecules on the “ceiling” of the chamber 211 (note “drops” of liquid such as drops 212). The spread or distribution on the ceiling 211 of condensing vapor molecules that were generated from a particular region of the liquid (e.g., those from region 210 ₁) is expected to be largely random. Hence, heat generated from a particular hot spot (e.g., hot spot 209 ₁) is expected to be randomly distributed across the ceiling 211 of the chamber.

Because the heat from a particular hot spot is uniformly distributed across the ceiling 211 of the chamber, the heat transfer from the particular hot spot is effectively distributed more uniformly to the external thermal interface. Importantly, this principle should apply to any hot spot irregardless of its location. As such, the approach of FIG. 2 should be capable of cooling any semiconductor chip irregardless of its design and corresponding hot spot location profile. Therefore, the use of custom designed cooling structures can be avoided.

In one embodiment, the chamber is first formed prior to its attachment to the semiconductor die 201. For example, walls 204, 205 are affixed to a first layer of material used for floor layer 203. Then, liquid is added to the chamber and lid 206 is applied over walls 204, 205 to seal the chamber. In an embodiment, the floor layer 203, walls 204, 205 and lid 206 are each comprised of Silicon (Si). In a further embodiment, a Si lid 206 is directly bonded to the Si chamber walls. The liquid may be comprised of various solutions such as water, alcohols, refrigerants or flourinerts such as FC-77.

In further or related embodiments the lid 206 has its exterior surface “processed” for efficient thermal coupling to an external thermal interface such as a heat sink. For example, the top surface of the lid 206 may be micro-machined or etched to effectively increase its surface area. Moreover or in the alternative, a layer 214 of thermally conductive material (e.g., metal) may be coated on the top surface of the lid 206. The coating 214 may be a multi-layer structure such as a first layer of metal beneath a second Indium alloy layer.

The studs 213 of floor layer 203 are comprised, in at least one embodiment, of carbon nanotubes. Here, it is generally understood in the art that carbon nanotubes may have different electrical properties. Examples include “conducting” and “semiconducting” carbon nanotubes. Generally, similar to other conducting materials, conducting carbon tubes have high thermal conductivities. Thus, in a further embodiment, the studs 213 of floor layer 203 include conducting carbon nanotubes. The use of conducting carbon nanotubes (as opposed to, for example, insulating carbon nanotubes) should enhance the transfer of heat from chamber floor layer 203 to liquid 208.

According to at least one approach, a chamber floor layer 203 with conducting carbon nanotubes 213 is formed by growing vertically oriented carbon nanotubes upon a substrate (such as a substrate comprised of Si). The substrate is used to implement chamber floor layer 203 and the vertically grown conducting carbon nanotubes correspond to studs 213.

Processes for vertically growing carbon nanotubes on a substrate (such as a substrate comprised of Si) have already been published in the art, see Z. Y. Juang, et al., 2004, “The effects of ammonia on the growth of large-scale patterned aligned carbon nanotubes using thermal chemical vapor deposition method”, Diamond and Related Materials, Vol, 13, no. 4-8 pp. 1203-1209; H. Konishi, et al., 2004, “Growth control of carbon nanotubes on silicon carbide surfaces using the laser irradiation effect”, Thin Solid Films, Vol. 464-465, pp. 295-298, and Ki-Hong Lee, et al., 2004, “Silicon enhanced carbon nanotube growth on nickel films by chemical vapor deposition” Solid State Communications, Vol. 129, No. 9, pp. 583-587, each of which presents different methods for the growth of carbon nanotubes on various types of surfaces.

According to one carbon nanotube growth technique, carbon nanotubes are spontaneously grown by placing a substrate coated with Nickel (Ni) into a plasma furnace containing ammonia gas and acetylene. A controlled electrical arc is passed through the sample, spontaneously causing growth of aligned nanotubes, see Z. F. Ren et al., 1998, “Synthesis of Large Arrays of Well-Aligned Carbon Nanotubes on Glass”, Science, Vol. 282, pp. 1105-1107.

Another interesting feature of using vertically oriented carbon nanotubes for studs 213 is the granularity at which the carbon nanotubes might be displaced on the surface of the chamber floor. To the extent that the heights reached by the vertically oriented carbon tubes are “short” and, as a consequence, their role of effectively increasing the surface area of the chamber floor layer 203 is less than impressive, note that that their lack of height is at least partially compensated for by the density at which they can be packed together. That is, given that carbon nanotubes are extremely small particles, they add to the effective surface area of the chamber floor layer 203 more by the number of surface perturbations that they effect rather than by the height of these perturbations.

FIG. 3 shows an alternate embodiment to that of FIG. 2 in which a liquid flow flows through the chamber 307. Here, an input liquid flow 320 is provided at a liquid flow input and an output liquid flow 321 is produced at a liquid flow output. The liquid flow through the chamber helps to more efficiently remove heat from the chamber and the semiconductor die. Here, approximately uniform heat removal can still be accomplished if the currents of liquid flow through the chamber is approximately uniform. In a further embodiment, the liquid flow is directed to some type of heat exchanging device (not shown in FIG. 3) that accepts warmed liquid from output 321 and converts the warmed liquid into cooled liquid. The liquid is retuned to the chamber 307 along with or from piping separate to that at which the input liquid flow 320 is provided.

Note that each of items 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, and 314 can behave similar to their respective counterparts 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, and 214 discussed with respect to FIG. 2.

FIG. 4 shows a cross section of a more complete packaged. According to the depiction of FIG. 4, a semiconductor die 401 is bonded to a substrate 430. The interface between the die 401 and the substrate 430 typically contains electrical input/output connections (I/Os) such as C4 connections formed on the die 401 and bonded to pads on the substrate 430. The external I/Os for the package can be implemented as leads or balls that emerge from the side or bottom of the substrate and that are electrically connected to the aforementioned pads by way of wiring formed within the substrate 430.

The die may be any type of die product such as a processor (e.g., general purpose processor, digital signal processor), memory device (e.g., Static Random Access Memory (SRAM) chip; Dynamic Random Access Memory (DRAM) chip) or non standard product offering Application Specific Integrated Circuit (ASIC) (i.e., a semiconductor chip not sold on the open market with its own part number or other identifier that identifies the chip alone) such as those commonly used to implement the switching and/or routing function within networking hardware equipment (e.g., switches, routers).

Atop the die 401, the complete chamber 400 containing liquid is shown. A cross section of the die 401 and chamber 400 may be as depicted in FIG. 2 or 3. The chamber 400 and die 401, in an embodiment may be Si—Si fusion bonded together. Atop the chamber 400 is an (optional) external thermal interface such as a heat sink. Recall from above that the top of the chamber 400 may have its exterior surface “processed” for efficient thermal coupling to the external thermal interface. For example, the top surface of the chamber may be micro-machined or etched to effectively increase its surface area. Moreover or in the alternative, a layer of thermally conductive material (e.g., metal) may be coated on the top surface of the chamber 400. The coating may be a multi-layer structure such as a first layer of metal beneath a second Indium alloy layer.

In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. 

1. An apparatus comprising: a chamber containing liquid, a side of said chamber thermally coupled to a semiconductor chip, said side of said chamber having thermally conductive carbon nanotubes oriented perpendicular to said side's surface, said carbon nanotubes to transfer heat drawn from said semiconductor chip into said liquid.
 2. The apparatus of claim 1 wherein said chamber is comprised of Silicon (Si) or Copper (Cu).
 3. The apparatus of claim 2 wherein said side is comprised of Si or Cu.
 4. The apparatus of claim 1 wherein another side of said of chamber is thermally coupled to an external thermal interface of a cooling system of which said chamber is part, said cooling system to transfer said heat to said external thermal interface.
 5. The apparatus of claim 4 wherein said external thermal interface comprises a heat sink.
 6. The apparatus of claim 4 wherein said external thermal interface comprises a heat exchanger.
 7. The apparatus of claim 4 wherein said another side is thermally coupled to said external interface by way of a liquid flow channel that carries warmed liquid from said chamber.
 8. The apparatus of claim 1 wherein said carbon nanotubes are electrically conductive carbon nanotubes.
 9. A method performed by a semiconductor chip's cooling system, comprising: transferring heat from said semiconductor chip to carbon nanotubes immersed in a liquid, said carbon nanotubes heating said liquid as a consequence; generating vapor from said liquid as a consequence of said heating; and, condensing said vapor to transfer at least a portion of said heat to said cooling system's external thermal interface.
 10. The method of claim 9 wherein said heating and generating takes place in a chamber, said carbon nanotubes attached to a side of said chamber, said side of said chamber thermally coupled to said semiconductor chip.
 11. The method of claim 10 wherein said heating, generating and condensing takes place in said chamber.
 12. The method of claim 10 further comprising drawing said liquid from said chamber.
 13. The method of claim 9 wherein said cooling system's external thermal interface further comprises a heat sink.
 14. The method of claim 9 wherein said carbon nanotubes are thermally and electrically conductive.
 15. A chamber comprising: walls; a floor sprouting vertically oriented carbon nanotubes, said walls standing on said floor, said floor thermally coupled to a semiconductor chip; a ceiling attached atop said walls, said ceiling thermally coupled to an external thermal interface; and, liquid within the volume bounded by said walls, floor and ceiling.
 16. The apparatus of claim 15 wherein said chamber is comprised of a material selected from the group consisting of: Silicon (Si); and Copper (Cu).
 17. The apparatus of claim 16 wherein said floor is comprised of a material selected from the group consisting of: Silicon (Si); and Copper (Cu).
 18. The apparatus of claim 14 wherein said external thermal interface comprises a heat sink.
 19. The apparatus of claim 15 wherein said carbon nanotubes are electrically and thermally conductive carbon nanotubes.
 20. The apparatus of claim 15 where the distance between said ceiling and said floor is 150 to 200 μm inclusive.
 21. The apparatus of claim 15 where said thermal coupling of said ceiling to said external thermal interface further comprises an etched surface of material, another surface of said material used to implement said ceiling, said surface and said another surface on opposite sides of said material.
 22. The apparatus of claim 15 where said thermal coupling of said ceiling to said external thermal interface further comprises a micro-machined surface of material, another surface of said material used to implement said ceiling, said surface and said another surface on opposite sides of said material.
 23. The apparatus of claim 15 where said thermal coupling of said ceiling to said external thermal interface further comprises a surface of material coated with metal, another surface of said material used to implement said ceiling, said surface and said another surface on opposite sides of said material.
 24. The apparatus of claim 23 further comprising an Indium alloy in contact with said metal.
 25. An apparatus, comprising: a) an SRAM semiconductor die; b) a chamber fixed to said die, said chamber comprising: walls; a floor sprouting vertically oriented carbon nanotubes, said walls standing on said floor, said floor thermally coupled to a semiconductor chip; a ceiling attached atop said walls, said ceiling thermally coupled to an external thermal interface; and, liquid within the volume bounded by said walls, floor and ceiling.
 26. The apparatus of claim 25 wherein said chamber is comprised of a material selected from the group consisting of: Silicon (Si); and Copper (Cu).
 27. The apparatus of claim 26 wherein said floor is comprised of a material selected from the group consisting of: Silicon (Si); and Copper (Cu).
 28. The apparatus of claim 24 wherein said external thermal interface comprises a heat sink.
 29. The apparatus of claim 25 wherein said carbon nanotubes are electrically and thermally conductive carbon nanotubes.
 30. The apparatus of claim 25 where the distance between said ceiling and said floor is 150 to 200 μm inclusive.
 31. The apparatus of claim 25 where said thermal coupling of said ceiling to said external thermal interface further comprises an etched surface of material, another surface of said material used to implement said ceiling, said surface and said another surface on opposite sides of said material.
 32. The apparatus of claim 25 where said thermal coupling of said ceiling to said external thermal interface further comprises a micro-machined surface of material, another surface of said material used to implement said ceiling, said surface and said another surface on opposite sides of said material.
 33. The apparatus of claim 25 where said thermal coupling of said ceiling to said external thermal interface further comprises a surface of material coated with metal, another surface of said material used to implement said ceiling, said surface and said another surface on opposite sides of said material.
 34. The apparatus of claim 33 further comprising an Indium alloy in contact with said metal.
 35. The apparatus of claim 25 wherein said chamber is Si—Si fusion bonded to said die. 